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The saphenous nerve arises from the distal femoral nerve as it finishes it’s branch distribution down the anterior thigh. It emerges from beneath the sartorius muscle in the medial distal upper leg and continues subcutaneously past the knee joint where it gives off the infrapatellar branch and then continues distally splitting off the Medial Cutaneous Nerves of the Leg.
The saphenous nerve follows the path of the greater saphenous vein and is almost unique for a nerve its size by being purely sensory in its nature. (the other such nerve group is the superficial cervical plexus)
Locating the Saphenous Nerve with Ultrasound
Start above the knee joint on the antero-medial aspect of the lower thigh, transverse to the long axis of the leg. Use plenty of ultrasound gel and move the probe up and down the leg, and in a circumferential orientation looking for the greater saphenous vein.
Scan depth on the probe should be set at a fairly shallow depth, above the muscle aponeurotic layer. Depth to the saphenous vein depends on the patient’s habitus. Patients with little subcutaneous fat will likely have the saphenous vein in the top 2 cms of depth. Scanning patients with large amounts of subQ should probably begin at a depth of 4 cms to find the depth to the muscle fascia.
The saphenous vein in a supine patient collapses readily with applied pressure so keep light pressure on the probe until you have identified the greater saphenous vein.
Be aware of the patient history -- the patient may be without a greater saphenous vein on the target side due to a previous surgery for vein ligation and stripping or being harvested as conduit for a CABG. If so, other landmarks may be used (discussed at a later date).
* In difficult cases, you may want to consider a light constrictive tourniquet around the mid thigh to help dilate and fix the saphenous vein, thus advantaging you in your search.
Below you will see 3 examples of scans of the saphenous nerve. Look them over and see if you can spot similarities in their arrangement.
Once you’ve located the greater saphenous vein, adjust you scan depth so that the vein, in its cross-section, is positioned in the top 1/3 to 1/4 of the screen. The saphenous nerve will lie posterior and deep to the vein. Just how much posterior and deep will vary from patient to patient. I apologize for this nebulous statement but we just haven’t surveyed all that many legs in this area as yet. Variations so far range from the nerve lying against the vein and running alongside it, to about 3 cm posterior and deep to the vein. On ultrasound you will see some horizontal lines of thick areolar connective tissue streaking the subcutaneous tissue, the nerve is usually separated from the vein by at least one (1) layer of this from the vein.
If you’re having trouble locating the nerve, try lowering the gain on the display and watching for the last structures to remain visible as the screen becomes dimmer. Depth of scan should be shallow enough not to show the muscle fascia. Move the probe along a few cms of the length of the vein and watch for structures that persist (meaning they don’t disappear). Change the angle at which the probe is held to the skin slightly back and forth, and clockwise and counter-clockwise to atttempt to find an angle that catches more of the reflected ultrasound energy.
Practice on yourself and others to see variations and get used to the depth and look of the vein and nerve.
I will post more technique here as they prove useful as well as more pictures.
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